可以在 Java 中实例化静态嵌套类吗? [英] Can a static nested class be instantiated in Java?

问题描述

从 Oracle 的 Java 教程我找到了这段文字:

From Oracle's Java tutorials I've found this text:

与类方法和变量一样，静态嵌套类与其外部类相关联.和静态类方法一样，静态嵌套类不能直接引用在其封闭类中定义的实例变量或方法——它只能通过对象引用来使用它们.

As with class methods and variables, a static nested class is associated with its outer class. And like static class methods, a static nested class cannot refer directly to instance variables or methods defined in its enclosing class — it can use them only through an object reference.

注意:静态嵌套类与其外部类(和其他类)的实例成员交互，就像任何其他顶级类一样.实际上，静态嵌套类在行为上是一个顶层类，为了方便打包，它嵌套在另一个顶层类中.

Note: A static nested class interacts with the instance members of its outer class (and other classes) just like any other top-level class. In effect, a static nested class is behaviorally a top-level class that has been nested in another top-level class for packaging convenience.

使用封闭类名访问静态嵌套类:

Static nested classes are accessed using the enclosing class name:

OuterClass.StaticNestedClass

OuterClass.StaticNestedClass

例如，要为静态嵌套类创建对象，请使用以下语法:

OuterClass.StaticNestedClassnestedObject =new OuterClass.StaticNestedClass();

我认为无法实例化静态类，所以我不太理解粗体的句子.

I thought it is not possible to instantiate a static class, so I don't really understand the sentence in bold.

你知道这是什么意思吗?

Do you have any idea what it means?

推荐答案

您要么像 kihero 所说的那样将 static 与 abstract 混淆，要么混淆了概念具有 static 方法的类(这只是一个碰巧具有静态方法的类).

You are either confusing static with abstract as kihero says, or you are muddling the concept with a class that has static methods (which is just a class that happens to have static methods).

静态嵌套类只是一个嵌套类，不需要其封闭类的实例.如果您熟悉 C++，那么 C++ 中的所有类都是静态"类.在 Java 中，嵌套类默认不是静态的(这种非静态变体也称为内部类")，这意味着它们需要外部类的实例，它们在隐藏字段中在幕后跟踪——但是这让内部类可以引用它们关联的封闭类的字段.

A static nested class is just a nested class that doesn't require an instance of its enclosing class. If you are familiar with C++, all classes in C++ are "static" classes. In Java, nested classes are not static by default (this non-static variety is also called an "inner class"), which means they require an instance of their outer class, which they track behind the scenes in a hidden field -- but this lets inner classes refer to fields of their associated enclosing class.

public class Outer {

    public class Inner { }

    public static class StaticNested { }

    public void method () {
        // non-static methods can instantiate static and non-static nested classes
        Inner i = new Inner(); // 'this' is the implied enclosing instance
        StaticNested s = new StaticNested();
    }

    public static void staticMethod () {
        Inner i = new Inner(); // <-- ERROR! there's no enclosing instance, so cant do this
        StaticNested s = new StaticNested(); // ok: no enclosing instance needed

        // but we can create an Inner if we have an Outer: 
        Outer o = new Outer();
        Inner oi = o.new Inner(); // ok: 'o' is the enclosing instance
    }

}

How to在静态方法中实例化非静态内部类

我实际上默认将所有嵌套类声明为静态，除非我特别需要访问封闭类的字段.

I actually declare all nested classes static by default unless I specifically need access to the enclosing class's fields.

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